The present invention is intended for use in telephone exchange modules of the kind which include a magazine which houses subscriber line circuits, a regional processor which forms the main control means of the line interface circuits, and associated circuits, such as ring generators, tone receivers, time selectors, for instance. The telephone exchange module is normally dependent on central group selectors which serve several telephone exchange modules. Each subscriber line circuit is connected to a respective subscriber over a telephone line. Normally, eight line circuits are combined on a circuit board, a so-called line board, although one line board may contain fewer or more line circuits. The remaining circuits of the module are also incorporated on circuit boards. The module as a whole thus comprises a large number of circuit boards which are arranged parallel with one another in a magazine. The circuit cards in a magazine are normally cooled by means of conventional convection-air-cooling systems. A local telephone exchange may comprise one or more such magazines. The magazines are placed in cabinets positioned in a room equipped with an air-conditioning system which functions to maintain the air temperature in the room within appropriate limits, these limits being selected so that the function of the telephone exchange module will be ensured irrespective of the ambient temperature. Energy is supplied to the magazine of a telephone exchange module from the mains network, via a conventional current supply unit comprising a rectifier and smoothing circuits. The local telephone exchange also includes a reserve current source, in the form of batteries. This reserve current source is used in the event of a mains failure, so that telephone traffic is able to continue unimpeded.
In unfavourable circumstances, for instance unsuitable positioning of the module or a failure in an air-conditioning system, the ambient temperature of the telephone exchange may rise to 40.degree.-50.degree. C., which can result in undesirable functional disturbances.
A telephone exchange, and not only a telephone exchange of the aforedescribed kind, but also telephone exchanges in general, must be dimensioned in accordance with its peak load. It is normally a requirement that telephone traffic shall be able to continue at peak load periods while maintaining a high sound quality, among other things. This creates dimensioning problems, among other things because the boards present in the magazine generate considerable heat during peak load periods. The development of heat in an underlying magazine influences, to some extent, the ambient temperature of an overlying magazine. Since the boards are densely packed side-by-side in a magazine, the heat generated adjacent one board will also influence the ambient temperature of adjacent boards. Such thermal influence is unfavourable and requires the use of circuit board components which are highly tolerable to heat, in order to ensure reliability in operation. This is expensive.
In order to dimension the telephone exchange so that it is able to manage all traffic during a peak load period, it is necessary to dimension the batteries so that they are able to deliver the power required to operate the exchange during a peak load period in the event of a mains failure. The batteries must therefore be dimensioned to deliver a high power output over a specified operating time, typically four hours. This makes the batteries expensive and bulky.
The temperature of a line board can also increase when a subscriber line is short, wherein the line has a low ohmic resistance and a high current passes through those circuits through which current is supplied to the line, hereinafter referred to as supply circuits. These supply circuits will therewith be heated. If, on the other hand, the subscriber line is long, its ohmic resistance will be high and a small current will pass through the supply circuits, which are thus not heated.
Another source resulting in the heating of line board components is the length of the call or conversation being made. The longer a call, the longer the line board components remain active and the more heat generated by these components.
Another source of heat is the number of lines which are active simultaneously on each line board.
German patent DE 25 51 916 relates to a device for reducing the loop current of a subscriber line in order to keep heat losses low. A control signal, derived from the loop current is used to select either a normal or a reduce line supply voltage. Should the loop current exceed a threshold value--as is the case for short subscriber lines, then the reduced supply voltage is selected and the loop current will drop. A reduced loop current is preferred in order to keep heat losses in the line circuit low thus allowing the use of cheap resistors in the line circuit. This selection is made at the occasion when the subscriber line is connected to the exchange. Once the supply voltage has been selected it is never changed.
This known device is of a static nature and will not account for varying operating conditions prevailing in the exchange such as for example an increase of the traffic intensity, an increase of the ambient temperature of the exchange or switching to battery operation of the exchange.